1. Field of the Invention
The invention relates to a conveyor device for conveying flat objects, which device comprises at least one pair of wheels having elastically deformable treads and disposed facing each other so as move a flat object by pinching and by friction.
The invention relates more particularly to a conveyor device suitable for moving a broad spectrum of flat mailpieces ranging from coupons of very small thickness (of about 0.5 millimeters (mm)) to flat mailpieces or “flats” of large format and of very large thickness (of about 30 mm).
2. Discussion of the Background Art
European Patent Document EP-1 194 249 discloses such a conveyor device that is part of a more complex system that can be referred to as a “synchronizer” in a mail sorting machine.
That synchronizer comprises a plurality of stages, each of which has two levels of motor-driven coupled wheels rotating at a speed that is controlled so as to synchronize the movement of a mailpiece (inserted between the facing coupled wheels) with the movement of a bin of a bin carrousel, into which bin said mailpiece is to be injected. In that known arrangement, each current mailpiece is pushed between the wheels of the inlet stage of the synchronizer by an unstacker member that is, in the example described, a perforated belt moving past a suction nozzle.
The synchronizer is controlled in a manner that takes account of the instant at which the current mailpiece goes past a sensor placed at the inlet of the synchronizer and of the instant at which a bin (into which the mailpiece is to be injected) goes past another sensor placed along the path along which the bins travel.
On the basis of a relatively constant speed of movement of the mailpiece between the outlet from the synchronizer and the point of injection into a bin, an acceleration curve is computed for the speed of the mailpiece in the synchronizer so as to make the passage of the bin coincide with the arrival of the mailpiece. In practice, at the outlet from the synchronizer, the mailpieces follow one another in series with constant spacing between their leading edges that corresponds to the spacing between four consecutive bins of the carrousel.
In that synchronizer, the inlet stage is made up of at least two facing wheels with elastically deformable treads that are of the same diameter. The two wheels are in contact with each other via the outside surfaces of their treads.
When a mailpiece is inserted on edge between the facing wheels of the inlet stage, the leading edge of said mailpiece deforms the treads of the facing wheels substantially on the line of contact of the treads. By being pushed back, the treads of the facing wheels exert a pinching force on the opposite surfaces of the mailpiece, that pinching force being distributed over almost the entire height of the mailpiece. Once the mailpiece is fully engaged between the two opposite wheels of the inlet stage, it is driven (with a certain amount of acceleration) by friction towards the wheels of the second stage and so on to the outlet from the synchronizer.
It has been observed that, for the thickest mailpieces, for which the treads deform to the largest extent, the insertion time required for inserting such mailpieces between the wheels of the inlet stage is also longer than for thinner mailpieces because such thick mailpieces take more time to pass through the synchronizer than such thinner mailpieces. Since the spacing between two consecutive mailpieces should be constant (or equal to a multiple of the constant spacing value), it has been observed that the synchronizer as described above delays the instant at which some of the thickest mailpieces are injected, e.g. by doubling or tripling the spacing between them and the mailpieces preceding them. That therefore affects the throughput of the mail sorting machine.